The combustion of an air-fuel mixture, for instance in an internal combustion engine (“ICE”) or a gas turbine, typically is initiated using a conventional spark ignition system. An electric arc discharge is generated in the air-fuel mixture, which heats the immediately surrounding air-fuel mixture to an extremely high temperature and causes electrons to escape from their nuclei, thereby creating a relatively small region of highly ionized gas. Combustion reaction(s) are then commenced in this small region of ionized gas. Under appropriate conditions the exothermic combustion reaction(s) heat the air-fuel mixture immediately surrounding the small region of ionized gas to cause further ionization and combustion. This chain-reaction process produces first a flame kernel in the combustion chamber of the ICE or gas turbine, and proceeds with a flame front moving through the combustion chamber until the air-fuel mixture is combusted.
In conventional spark ignition systems the electric arc discharge is created when a high voltage DC electric potential is applied across two electrodes in the combustion chamber. A relatively short gap is formed between the electrodes, such that the high voltage potential causes a strong electric field to develop between the electrodes. This strong electric field causes dielectric breakdown in the gas between the electrodes. The dielectric breakdown commences when seed electrons, which are naturally present in the air-fuel gas, are accelerated to a highly energetic level by the strong electric field. More particularly, a seed electron is accelerated to such a high energy level that when it collides with another electron in the air-fuel gas, it knocks that electron free of its nucleus resulting in two lower energy level free electrons and an ion. The two lower energy level free electrons are then in turn accelerated by the electric field to a high energy level and they, too, collide with and free other electrons in the air-fuel gas. This chain reaction results in an electron avalanche, such that a large proportion of the air-fuel gas between the electrodes is ionized into charge carrying constituent particles (i.e., ions and electrons). With such a large proportion of the air-fuel gas ionized, the gas no longer has dielectric properties but acts rather as a conductor and is called plasma. A high current passes through a thin, brilliantly lit column of the ionized air-fuel gas (i.e., the arc) from one electrode to the other until the charge built up in the ignition system is dissipated. Because the gas has undergone complete dielectric breakdown, when this high current flows there is a low voltage potential between the electrodes. The high current causes intense heating—up to 30,000° F.—of the air-fuel gas immediately surrounding the arc. It is this heat which sustains the ionization of the air-fuel mixture long enough to initiate combustion.
Unfortunately, conventional spark ignition systems have a number of drawbacks and limitations. In an ICE the electrodes of the spark ignition system are typically part of a spark plug, which penetrates into the combustion chamber. The extreme heat that is produced by the electric arc during ignition damages the electrodes over time. Also, because of its reliance upon creating heat to ionize the air-fuel mixture, the maximum energy output of a conventional spark ignition system is limited by the amount of heat the electrodes can sustain. Further, a recent trend is to dilute the air-fuel combustible mixture by increasing the air/fuel ratio, or by increasing the level of exhaust gas recirculation (EGR), thereby enabling operation at higher compression ratios and loads and achieving cleaner and more efficient combustion. Unfortunately, increased dilution levels give rise to problems relating to both ignition and flame propagation in conventional spark ignition systems. As such, a more robust ignition system is required.
Another method for igniting the air-fuel mixture in a combustion chamber of an ICE or a gas turbine is by way of a corona discharge. In this type of system an igniter having center electrode held by an insulator is used, which forms a capacitance together with an outer conductor enclosing the insulator or with the walls of the combustion chamber at ground potential, as counter electrode. The insulator enclosing the center electrode and the combustion chamber, with the contents thereof, act as a dielectric. The capacitance so-formed is a component of an electric oscillating circuit, which is excited using a high-frequency voltage that is created, for example, using a step-up transformer. The transformer interacts with a switching device, which applies a specifiable DC voltage to the primary windings, and produces a sinusoidal alternate current wave in the secondary winding. The secondary winding of the transformer supplies a series oscillating circuit having the capacitance formed by the center electrode and the walls of the combustion chamber. The frequency of the alternating voltage that excites the oscillating circuit is controlled such that it is as close as possible to the resonance frequency of the oscillating circuit. The result is a voltage step-up between the ignition electrode and the walls of the combustion chamber within which the ignition electrode is disposed. Under these conditions, a corona discharge can be created in the combustion chamber.
Unfortunately, after ignition and during combustion the radicals that are produced in the combustion zone cause the capacitance of the combustion zone and the system resonant frequency to change. As such, the corona formation must be controlled during the ignition process in order to achieve optimal ignition results and to prevent the occurrence of arcing. Known approaches for controlling the corona formation and for preventing the occurrence of arcing involve shifting the operating frequency away from the resonant frequency to result in a drop in the high voltage at the ignition electrode to prevent further arcing. Subsequently, the voltage applied to the primary winding can be decreased, then the operating frequency can be returned to the resonant frequency in order to improve efficiency. Such an approach is complex and inefficient.
It would be beneficial to provide a corona ignition system and related methods that overcome at least some of the above-mentioned drawbacks and limitations of known systems.